"Missing" shipment is a paper note in the Fallout: New Vegas add-on Lonesome Road.
Location
It can be found in the gun case with the Red Victory Grenade Rifle in a ruined tent on a cliff, west side of Dry Wells.
Transcript
General,
We found a seized shipment of Chinese "Red Victory" model grenade launchers and thought you might like to have one for yourself. They're quicker to reload then our "Great Bears," but they don't quite pack the same punch. Still, it'll be nice to give those bastards a taste of their own medicine if it comes to that, right?
Notes
Despite the note mentioning that Red Victory Grenade Rifles reload faster than Great Bear Grenade Rifles, the only obtainable versions of the weapons share the same reload time in-game.